1. Field of the Invention
The present invention relates to a vacuum pressure booster used for the boosting operation of a master cylinder of a brake of a vehicle. More particularly, the present invention relates to improvements in a vacuum pressure booster having: a booster shell; a booster piston accommodated inside the booster shall and partitioning the interior of the booster shell into a front side vacuum pressure chamber communicating with a vacuum pressure source and a rear side operation chamber; a valve cylinder communicating with the booster piston.
The valve cylinder includes: a valve piston fitted into the valve cylinder to be slidable in a forward and rearward direction of the valve cylinder; an input rod coupling with the valve piston at a front end thereof; a control valve switching communication of the operation chamber with the vacuum pressure chamber and with air in accordance with a forward and rearward movement of the input rod between the valve piston and the valve cylinder; and an input return spring for pushing the input rod backward.
The control valve includes: an annular vacuum pressure introducing valve seat formed in the valve cylinder; an atmosphere introducing valve seat formed in the valve piston and arranged inside the vacuum pressure introducing valve seat; a valve body including: an annular attaching bead portion airtightly attached to the valve cylinder; an expansion cylinder portion extending in the axial direction from the attaching bead portion; and an annular valve portion communicating with a forward end portion of the expansion cylinder portion and opposed to the vacuum pressure introducing valve seat and the atmosphere introducing valve seat so as to seat thereon; and a valve spring for pushing the valve portion so as to seat on the vacuum pressure introducing valve seat and the atmosphere introducing valve seat.
A first port communicates with the vacuum pressure chamber is opened on the outer circumferential side of the vacuum pressure introducing valve seat, a second port communicates with the operation chamber is opened between the vacuum pressure introducing valve seat and the atmosphere introducing valve seat in such a manner that the inner circumferential side of the valve portion is communicated with the atmosphere.
2. Description of the Related Art
The above vacuum pressure booster has already been well known as disclosed, for example, in JP-UM-B-58-48923.
In the above conventional vacuum pressure booster, when an attaching bead portion of the valve body is attached to the valve cylinder, the attaching bead portion is tightly engaged on an inner circumferential face of the valve cylinder, and an annular valve holder is tightly engaged on an inner circumferential face of the bead portion. Therefore, it is impossible for a diameter of the attaching bead portion to be smaller than the inner diameter of the valve cylinder. However, when consideration is given to the airtightness of the attaching portion of the bead portion, it is preferable that the diameter of the attaching bead portion is reduced as small as possible so that the attaching area can be reduced.